A spaceborne ultra-high-resolution SAR imaging method with real-time tropospheric correction

Abstract

This invention proposes a spaceborne ultra-high-resolution SAR imaging method with real-time tropospheric correction, which meets the requirements of high timeliness and high precision SAR imaging processing without being limited by the imaging area. Specifically, a corrected radiometer capable of covering the SAR observation field of view is used to simultaneously measure atmospheric temperature and water vapor pressure; the measurement data and SAR echo data are simultaneously transmitted to the ground to obtain the wet tropospheric zenith delay; using the total tropospheric delay estimation model, the total tropospheric zenith delay is obtained, and the slant range delay caused by the troposphere is further obtained; the slant range delay is used as a correction term to correct the SAR slant range model; in the ultra-high-resolution SAR imaging processing on the ground, the imaging algorithm adopts the slant range corrected by the real-time tropospheric correction, and finally obtains the ultra-high-resolution SAR image.

Description

Technical Field

[0001] This invention belongs to the field of radar imaging, and in particular relates to a spaceborne ultra-high resolution SAR imaging method with real-time tropospheric correction. Background Technology

[0002] The troposphere in the atmosphere is the main factor affecting electromagnetic wave transmission. Electromagnetic waves undergo significant refraction during propagation within it. Because the refractive index of electromagnetic waves varies with atmospheric altitude, the curved refraction path causes slant range errors, resulting in varying slant range errors over time within the synthetic aperture time (SAP). This leads to excessive residual errors in range migration correction, causing azimuth imaging defocus. As spaceborne SAR imaging evolves from low-frequency (L-band) to high-frequency (Ka-band), imaging resolution continuously improves from the tens of meters to the decimeter level. This places increasingly stringent requirements on slant range error control (at the decimeter level), thus creating an urgent need for high-precision atmospheric delay correction methods.

[0003] Existing methods for atmospheric delay correction in spaceborne SAR mainly fall into two categories: one is to estimate the atmospheric delay of the imaging area using weather forecast models. Patent "Method and System for InSAR Atmospheric Delay Correction Based on ECMWF" (CN201910257685) calculates the total zenith delay of the imaging area based on ECMWF weather forecast data and DEM data, and obtains the atmospheric delay phase value of each imaging point on the SAR image by constructing a layered interpolation model, thus improving the accuracy of atmospheric delay correction. However, this type of method faces problems such as inconsistent observation times, spatial resolution mismatch, and susceptibility to cloud influence, reducing its applicability. The other category uses GNSS tomography to estimate the imaging path delay. Patent "A GNSS Tomography-Assisted InSAR Atmospheric Delay Correction Method" (CN202011502257.9) uses all-weather GNSS data to obtain an atmospheric delay phase map synchronized with the SAR image, but this requires multiple GNSS receivers to be set up in the imaging area, imposing limitations on the imaging area. Summary of the Invention

[0004] Given the high requirements of spaceborne ultra-high resolution SAR for co-path tropospheric delay measurement, this invention proposes a spaceborne ultra-high resolution SAR imaging method with real-time tropospheric correction, which meets the requirements of high timeliness and high precision SAR imaging processing without being limited by the imaging area.

[0005] Satellite radar signals are affected by changes in refractive index as they pass through the troposphere, causing the propagation path to bend and resulting in a delay in actual slant range compared to the theoretical value. To ensure ultra-high resolution imaging, slant range accuracy often reaches the decimeter level, thus requiring high-precision tropospheric delay data. Tropospheric delay mainly consists of two parts: one is the wet tropospheric delay caused by water vapor, liquid water in clouds, etc., and the other is the dry tropospheric delay caused by dry gases in the atmosphere. The wet tropospheric delay is obtained through synchronous measurement using a calibrated radiometer, while the dry tropospheric delay can be accurately measured using a model, with an accuracy on the order of 0.05 centimeters.

[0006] Specifically, this invention employs a calibrated radiometer capable of covering the SAR observation field of view to measure, in real time, the atmospheric temperature T(t,η) and water vapor pressure e(t,η) above the observation scene at different observation angles η within the SAR working time range. The calibrated radiometer measurement data and SAR echo data are simultaneously transmitted to the ground, and after ground processing, the moist tropospheric zenith delay is obtained. Using the total tropospheric delay estimation model, the total tropospheric zenith delay at different imaging times t is obtained. The total tropospheric delay estimation model is the sum of the dry tropospheric delay and the average of the moist tropospheric zenith delay at different observation angles η. Furthermore, by mapping the total tropospheric zenith delay to the line-of-sight delay, the tropospheric slant range delay RTD(t) is obtained. The slant range delay RTD(t) is used as a correction term to correct the SAR slant range model. In the ultra-high resolution SAR imaging processing on the ground, the imaging algorithm uses the slant range corrected by the tropospheric real time, and finally obtains the ultra-high resolution SAR image.

[0007] Furthermore, the method for calibrating the radiometer is as follows: during the process of the antenna rotating one revolution along the direction of the track, the atmospheric observation and calibration are completed by measuring the cold air background, blackbody and ground target.

[0008] Furthermore, during ground processing, the Saastamoinen model can be used to input the atmospheric temperature and water vapor pressure measured in real time by the calibrated radiometer to obtain the wet tropospheric zenith delay.

[0009] Furthermore, the formula for calculating dry tropospheric delay is as follows:

[0010]

[0011] Where P0 represents sea surface pressure. The latitude of the center of the observed scene.

[0012] Furthermore, during imaging processing, the SAR slant range model is subjected to Taylor expansion, and the Taylor expansion order is retained according to the accuracy requirements.

[0013] Furthermore, the second-order expanded SAR slant range model is as follows:

[0014]

[0015] Among them, R c λ is the slant range corresponding to the center of the observed scene, λ is the radar wavelength, and t is the slant range. c The moment when SAR images the center of the scene. f is the Doppler center frequency. R The Doppler frequency modulation slope;

[0016] The corrected second-order SAR slant range model is as follows:

[0017] R'(t) = R(t) + RTD(t).

[0018] Beneficial effects

[0019] (1) This invention proposes an on-orbit calibrated radiometer to measure the liquid water and water vapor content of the atmosphere within the working angle range of the SAR antenna, which is used to correct the tropospheric / atmospheric delay in the ground processing of high-resolution SAR satellites, thus ensuring the accuracy of high-frequency and ultra-high-resolution processing.

[0020] (2) This invention avoids excessive reliance on high-precision ground weather forecast information and greatly improves the system's autonomous closed-loop performance;

[0021] (3) The present invention transmits tropospheric delay measurement data and payload echo data simultaneously, further improving the timeliness of SAR data processing. Attached Figure Description

[0022] Figure 1 Flowchart of the method of this invention;

[0023] Figure 2 Schematic diagram of simultaneous observations by SAR and calibrated radiometer;

[0024] Figure 3 The relationship between latitude and dry tropospheric delay;

[0025] Figure 4 The relationship between atmospheric pressure and dry tropospheric delay. Detailed Implementation

[0026] To make the objectives, technical solutions, and advantages of this disclosure clearer, the following detailed description is provided in conjunction with specific embodiments and the accompanying drawings.

[0027] This invention employs a calibrated radiometer to measure the liquid water and water vapor content in the atmosphere within the operational field of view of a SAR antenna, and generates atmospheric delay data in orbit over a long period. When the SAR satellite begins operation, the atmospheric experimental data measured by the calibrated radiometer is packaged together with the SAR echo data and transmitted to the ground. In the ground system, the on-orbit measured atmospheric calibration data is incorporated into a high-precision atmospheric delay estimation model, further improving the spatiotemporal accuracy of atmospheric delay correction. The measured fine tropospheric delay is then provided to the SAR ground system for high-precision SAR imaging processing, effectively improving the accuracy of existing SAR imaging processing.

[0028] The on-orbit tropospheric real-time correction process is as follows:

[0029] (1) Same distance observation

[0030] This invention employs a high-precision, calibrated radiometer capable of covering the SAR observation field of view to simultaneously measure atmospheric temperature T(t,η) and water vapor pressure e(t,η) at different observation angles η within the SAR operating time range and above the observation scene. A schematic diagram of the simultaneous observation is shown below. Figure 2 As shown.

[0031] The calibrated radiometer receiver employs a full-power microwave radiometer design, which is simple in structure, easy to implement, and highly reliable. Because the calibrated radiometer measures the observed target throughout the entire integration time, it is also the most sensitive radiometer. Onboard calibration uses a two-point calibration scheme: a high-temperature point for blackbody radiation and a low-temperature point for cold-space background radiation. Frequency band selection for the calibrated radiometer requires thorough justification, prioritizing non-SAR imaging bands whenever possible. The operating mode employs a cross-track circular scanning method.

[0032] The calibration of the radiometer employs a comprehensive calibration scheme using a rotating antenna. Specifically, the antenna rotates one revolution along the track direction, measuring the cold air background, blackbody, and ground targets to complete the observation and calibration of the atmosphere.

[0033] (2) Estimation of wet tropospheric delay

[0034] The calibration radiometer measurement data and SAR echo data are simultaneously transmitted to the ground. The high-precision Saastamoinen model is used to obtain the wet tropospheric zenith delay during ground processing.

[0035] Compared to the influence of the dry troposphere, the absolute value of the wet troposphere is smaller, but it changes rapidly over time. Using a calibrated radiometer, atmospheric temperature T(t,η) and water vapor pressure e(t,η) can be measured in real time. Substituting these real-time measurements into the Saastamoinen model, the zenith delay of the wet troposphere can be estimated. Specifically, at a given imaging time t and a calibrated radiometer observation angle η, the zenith delay D of the wet troposphere is...wet (t,η) is represented as

[0036]

[0037] (3) Dry tropospheric delay estimation

[0038] For the dry troposphere, when hydrostatic equilibrium and the ideal gas law hold, the vertical integral of the distance delay is only a function of surface pressure, and the atmospheric pressure generated by dry gases accounts for more than 99% of the total atmospheric pressure, so the difference between the two can be ignored. The influence of the dry troposphere is almost constant. In this case, the expression for the dry troposphere's distance delay integral can be approximately expressed as the relationship between sea surface pressure P0 and gravitational acceleration g:

[0039]

[0040] Among them, gravitational acceleration with latitude The relation is:

[0041]

[0042] g0 = 980.6 cm / sec 2 This is the standard reference value for gravitational acceleration. mbar (millibar) is a common term for air pressure; in the International System of Units (SI), 1 mbar = 1 hPa (hectopascal). Taylor expansion, retaining the latitude-dependent maximum term, yields the dry tropospheric delay as follows:

[0043]

[0044] P0 is typically around 1000mb, with correction values ​​between -2.23 and -2.35 meters. Currently, the global atmospheric pressure accuracy is better than 3mb, so the standard deviation after dry tropospheric delay correction is better than 0.05cm.

[0045] (4) Estimation of total tropospheric delay, and then obtaining the tropospheric slant delay RTD(t).

[0046] At a certain imaging time t, the total tropospheric zenith delay is the sum of the average values ​​of the dry tropospheric delay and the wet tropospheric delay at different angles, as shown in the following formula.

[0047]

[0048] The calibration radiometer used a total of N observation angles.

[0049] SARs all operate in side-looking mode. The slant range delay caused by the troposphere is obtained by mapping the zenith delay to the line-of-sight delay, as detailed below.

[0050]

[0051] Where θ is the incident angle of the radar signal.

[0052] (5) Real-time slope distance correction

[0053] The systematic delay of spaceborne SAR is an inherent delay within the SAR payload, and it is necessary to accurately account for the systematic delay of SAR during the imaging processing and SAR geometric calibration processes.

[0054] For high-resolution SAR satellites, the systematic delay of SAR is calibrated through satellite-to-ground geometric calibration. During geometric calibration, to more accurately measure the systematic delay, it is necessary to measure local atmospheric composition and pressure parameters at the calibration field to estimate the tropospheric delay. Using this method, the tropospheric delay during geometric calibration can be measured simultaneously using an onboard radiometer. Therefore, the actual SAR systematic delay is the difference between the SAR ground-measured systematic delay R0 and the tropospheric slant range delay, where the tropospheric slant range delay is the time average of the slant range delay measured by the radiometer within the synthetic aperture time. The corrected SAR systematic delay is simply expressed as follows:

[0055]

[0056] The corrected SAR systematic delay is primarily used for SAR geometric calibration. This simplifies the on-orbit calibration process and improves the geometric positioning accuracy of spaceborne SAR systems.

[0057] In SAR imaging, the slant range is the distance between the antenna's equivalent phase center and the ground observation point. Typically, during image processing, a Taylor expansion of the SAR slant range model is performed. Different expansion orders can be retained depending on the actual accuracy requirements. Currently, we take retaining the quadratic term as an example (in addition, the third order and above can be retained as needed). Without considering tropospheric delay, the SAR slant range model is as follows:

[0058]

[0059] Where R(t) is the slant range corresponding to the azimuth direction of SAR imaging at time t, R c λ is the slant range corresponding to the center of the observed scene, λ is the radar wavelength, and t is the slant range. c The moment when SAR images the center of the scene. f is the Doppler center frequency. R This represents the Doppler frequency modulation slope.

[0060] Taking into account the tropospheric delay, the slant range model is further modified to...

[0061]

[0062] (6) Ultra-high resolution SAR imaging processing

[0063] In ultra-high resolution SAR imaging processing, the slant range model after real-time correction of the troposphere (Equation (9)) is used in the imaging algorithm to realize ultra-high resolution SAR imaging.

[0064] Meanwhile, when performing azimuth imaging processing, a matched filter (Equation (7)) is generated using the corrected systematic time delay, thereby ensuring the high-precision slant range requirement of ultra-high resolution SAR imaging processing.

Claims

1. A spaceborne ultra-high resolution SAR imaging method with real-time tropospheric correction, characterized in that: A calibrated radiometer capable of covering the SAR observation field of view is used to measure the atmospheric temperature T(t,η) and water vapor pressure e(t,η) above the observation scene at different observation angles η within the SAR working time range in real time. The calibrated radiometer measurement data and SAR echo data are transmitted to the ground through a dual-path synchronous link. The ground is processed by the Saastamoinen model to obtain the wet tropospheric zenith delay. Using the total tropospheric delay estimation model, the total tropospheric zenith delay at different imaging times t is obtained. The total tropospheric delay estimation model is the sum of the dry tropospheric delay and the average of the wet tropospheric zenith delay at different observation angles η. Furthermore, by mapping the total tropospheric zenith delay to the line-of-sight delay, the tropospheric-induced slant range delay RTD(t) is obtained. The slant range delay RTD(t) is used as a correction term to correct the SAR slant range model. In ultra-high resolution SAR imaging processing on the ground, the imaging algorithm uses the slant range corrected by the tropospheric real time, and a single imaging can achieve decimeter-level accuracy, ultimately obtaining an ultra-high resolution SAR image.

2. The spaceborne ultra-high resolution SAR imaging method with real-time tropospheric correction according to claim 1, characterized in that: The method for calibrating a radiometer is as follows: as the antenna rotates one revolution along the direction of the track, the atmospheric observation and calibration are completed by measuring the cold air background, blackbody, and ground targets.

3. A spaceborne ultra-high resolution SAR imaging method with real-time tropospheric correction according to claim 1 or 2, characterized in that: For ground processing, the wet tropospheric zenith delay can be obtained by inputting the atmospheric temperature and water vapor pressure measured in real time by the calibrated radiometer into the Saastamoinen model.

4. The spaceborne ultra-high resolution SAR imaging method with real-time tropospheric correction according to claim 3, characterized in that: The formula for calculating dry tropospheric delay is as follows: Where P0 represents sea surface pressure. Indicates the latitude of the center of the observation scene.

5. The spaceborne ultra-high resolution SAR imaging method with real-time tropospheric correction according to claim 4, characterized in that: During imaging processing, the SAR slant range model is subjected to Taylor expansion, and the Taylor expansion order is retained according to the accuracy requirements.

6. The spaceborne ultra-high resolution SAR imaging method with real-time tropospheric correction according to claim 5, characterized in that: The SAR slant range model under second-order expansion is as follows. Among them, R c λ is the slant range corresponding to the center of the observed scene, λ is the radar wavelength, and t is the slant range. c The moment when SAR images the center of the scene. f is the Doppler center frequency. R The Doppler frequency modulation slope; The corrected second-order SAR slant range model is as follows: R'(t) = R(t) + RTD(t).

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